HERMES Collaboration Research Articles

Investigations into the flavor dependence of partonic transverse momentum

Recent experimental data on semi-inclusive deep-inelastic scattering from the HERMES collaboration allow us to discuss for the first time the flavor dependence of unpolarized transverse-momentum dependent distribution and fragmentation functions. We find convincing indications that favored fragmentation functions into pions have smaller average transverse momentum than unfavored functions and fragmentation functions into kaons. We find weaker indications of flavor dependence in the distribution functions.

Beam spin asymmetries of charged and neutral pion production in semi-inclusive DIS

We present a study on the beam single spin asymmetries $A_{LU}^{\sin\phi_h}$ of $\pi^+$, $\pi^-$ and $\pi^0$ production in semi-inclusive deep inelastic scattering process, by considering Collins effect and the $g^\perp D_1$ term simultaneously. We calculate the twist-3 distributions $e(x, \bm k_T^2)$ and $g^\perp(x,\bm k_T^2)$ for the valence quarks inside the proton in a spectator model. We consider two different options for the form of diquark propagator, as well as two different choices for the model parameters in the calculation. Using the model results, we estimate the beam spin asymmetries $A_{LU}^{\sin\phi_h}$ for the charged and neutral pions and compare the results with the measurement from the HERMES Collaboration. We also make predictions on the asymmetries at CLAS with a $5.5 \,\textrm{GeV}$ beam using the same model results. It is found that different choices for the diquark propagator will not only lead to different expressions for the distribution functions, but also result in different sizes of the asymmetries. Our study also shows that, although the spectator model calculation can describe the asymmetries for certain pion production in some kinematic regions, it seems difficult to explain the asymmetries of pion production for all three pions in a consistent way from the current versions.

QCD Analysis of the semi-inclusive HERMES and COMPASS data

The first moments of polarized valence PDFs, responsible for the respective contributions to the nucleon spin, are extracted in NLO QCD from the data of COMPASS and HERMES collaborations on the semi-inclusive difference asymmetries. To this end the new method of QCD analysis (alternative to the usual global fit analysis) is applied. Using the obtained results on valence PDFs the first moments of polarized sea PDFs are reconstructed. They turn out surprisingly small: compatible with zeros within the errors.

Beam single spin asymmetry of neutral pion production in semi-inclusive deep inelastic scattering

We study the beam spin asymmetry $A_{LU}^{\sin\phi_h}$ in semi-inclusive $\pi^0$ electroproduction contributed by the T-odd twist-3 distribution function $g^\perp(x,k_T^2)$. We calculate this transverse momentum dependent distribution function for the $u$ and $d$ quarks inside the proton in a spectator model including the scalar and the axial-vector diquark components. Using the model results, we estimate the asymmetry $A_{LU}^{\sin\phi_h}$ in the $e p \rightarrow e^\prime \pi^0 X$ process in which the lepton beam is longitudinally polarized. The model prediction is compared with the data measured by the CLAS and the HERMES Collaborations and it is found that our numerical results agree with experimental data reasonably. Especially, our results can well describe the CLAS data at the region where the Bjorken $x$ and the pion transverse momentum is not large. We also make a prediction on the asymmetry $A_{LU}^{\sin\phi_h}$ in a $\pi^0$ electroproduction at CLAS12 using the same model calculation.

I – Experimental investigation of transverse spin asymmetries in μ-p SIDIS processes: Collins asymmetries

The COMPASS Collaboration at CERN has measured the transverse spin azimuthal asymmetry of charged hadrons produced in semi-inclusive deep inelastic scattering using a 160 GeV μ+ beam and a transversely polarised NH3 target. The Collins asymmetry of the proton was extracted in the Bjorken x range 0.003<x<0.7. These new precise measurements confirm previous measurements from the COMPASS and HERMES Collaborations, which exhibit a definite effect in the valence quark region. The asymmetries for negative and positive hadrons are similar in magnitude and opposite in sign. They are compatible with model calculations in which the u-quark transversity is opposite in sign and somewhat larger than the d-quark transversity distribution function. The asymmetry is extracted as a function of Bjorken x, the relative hadron energy z and the hadron transverse momentum pTh. The high statistics and quality of the data also allow for more detailed investigations of the dependence on the kinematic variables. These studies confirm the leading-twist nature of the Collins asymmetry.

Hadron production in lepton-nucleus interactions at high energies: Monte carlo generator HARDPING 2.0

Hadron production in lepton-nucleus interactions at high-energies is considered in framework of developing Monte Carlo event generator HARDPING (HARD Probe INteraction Generator). Such effects as formation length, energy loss and multiple rescattering for produced hadrons and their constituents are implemented into the HARDPING 2.0. Available data from HERMES collaboration on hadron production in lepton-nucleus collisions are described by the present version of the HARDPING generator in a reasonable agreement.

Constraints on the [formula omitted] generalized parton distribution from deep virtual Compton scattering measured at HERMES

We have analyzed the longitudinally polarized proton target asymmetry data of the Deep Virtual Compton process recently published by the HERMES Collaboration in terms of Generalized Parton Distributions. We have fitted these new data in a largely model-independent fashion and the procedure results in numerical constraints on the H˜Im Compton Form Factor. We present its t- and ξ-dependencies. We also find improvement on the determination of two other Compton Form Factors, HRe and HIm.

Boer-Mulders effect in unpolarized SIDIS: An analysis of the COMPASS and HERMES data on thecos2ϕasymmetry

We present a phenomenological analysis of the $\cos 2 \phi$ asymmetry recently measured by the COMPASS and HERMES collaborations in unpolarized semi-inclusive deep inelastic scattering. In the kinematical regimes explored by these experiments the asymmetry arises from transverse-spin and intrinsic transverse-momentum effects. We consider the leading-twist contribution, related to the so-called Boer-Mulders transverse-polarization distribution $h_1^{\perp}(x, k_T^2)$, and the twist-4 Cahn contribution, involving unpolarized transverse-momentum distribution functions. We show that a reasonably good fit of the data is achieved with a Boer-Mulders function consistent with the main theoretical expectations. Our conclusion is that the COMPASS and HERMES measurements represent the first experimental evidence of the Boer-Mulders effect in SIDIS.

Generalized Parton Distributions from Deeply Virtual Compton Scattering at HERMES

The HERMES Collaboration has recently published a set of (correlated) beam charge, beam spin and target spin asymmetries for the Deeply Virtual Compton Scattering (DVCS) process. This reaction allows in principle to access the Generalized Parton Distributions (GPDs) of the nucleon. We have fitted, in the QCD leading-order and leading-twist handbag approximation, but in a model-independent way, this set of data and we report our results for the extracted Compton form factors. In particular, we are able to extract constrains on the H GPD.

Nuclear medium effects on hadronization measured by the Hermes collaboration

The effects of the cold nuclear medium on hadron multiplicities in semi-inclusive deep inelastic scattering was studied extensively and is presented together with the first direct measurement of p t -broadening in cold nuclear matter. The data have been accumulated by the Hermes experiment at Desy , in which the Hera 27.6 GeV lepton beam scattered off several nuclear gas targets. These analysis provide precious information concerning the space-time evolution of hadronization.

Cos2ϕasymmetries in unpolarized semi-inclusive DIS

We use the Boer-Mulders functions parameterized from unpolarized p+D Drell-Yan data by the FNAL E866/NuSea Collaboration combined with recently extracted Collins functions to calculate the cos2{phi} asymmetries in unpolarized semi-inclusive deeply inelastic scattering (SIDIS) processes both for ZEUS at the Hadron Electron Ring Accelerator (HERA) and Jefferson Lab (JLab) experiments and to compare our results with their data. We also give predictions for the cos2{phi} asymmetries of SIDIS in the kinematical regime of HERMES Collaboration, and the forthcoming JLab experiments. We predict that the cos2{phi} asymmetries of semi-inclusive {pi}{sup -} production are somewhat larger than that of {pi}{sup +} production. We suggest the measurement of these two processes separately, which will provide more detailed information on the Boer-Mulders functions as well as on the Collins functions.

HERMES measurement of the collins and sivers asymmetries from a transversely polarized hydrogen target

In 2005 the HERMES collaboration published first evidence for azimuthal single-spin asymmetries in semi-inclusive production of charged pions on a transversely polarised hydrogen target. The distinctive Fourier components of the measured asymmetries caused by both the Collins and the Sivers mechanisms provide signals for previously unmeasured quantities: the transversity quark distribution in conjunction with the Collins fragmentation function (Collins mechanism) and the Sivers parton distribution (Sivers mechanism). The transversity distribution will provide a detailed insight into the relativistic nature of the quarks inside the nucleon. Recent theoretical work demonstrated a relation between the Sivers function and the contribution of the orbital angular momentum of quarks within the nucleon. The results presented for charged pions have a much higher statistical significance than the published data. In addition the Collins and Sivers asymmetries for neutral pions and charged kaons are provided.

The HERMES Recoil Detector

The HERMES Collaboration installed a new Recoil Detector to upgrade the existing spectrometer to study hard exclusive processes which provide access to generalised parton distributions (GPDs) and hence to the orbital angular momentum of quarks. The HERMES Recoil Detector mainly consists of three components: a silicon detector surrounding the target cell inside the beam vacuum, a scintillating fibre tracker and a photon detector with three layers of tungsten and scintillator bars in three different orientations. All three detectors are located inside a solenoidal magnet which provides a 1T longitudinal magnetic field. The Recoil Detector was installed in January 2006 and data taking will last until July of 2007.

Goodbye to large ΔG? Direct estimate of gluon polarization in the nucleon

We make a first crude direct estimate of the net gluon polarization in the proton, Δ G , combining data on the asymmetries in high- p T hadron production from the HERMES, SMC and COMPASS collaborations. Although these data sample a restricted range of x , they provide no hint that Δ G is large. Fixing the normalizations of different theoretical parametrizations using the hadron asymmetry data, we find typical central values of Δ G ∼ 0.5 , with uncertainties of similar magnitude. Values of Δ G ≥ 2 are disfavoured by Δ χ 2 ∼ 9 to 20 , depending on the parametrization used.

DIRECT ESTIMATE OF THE GLUON POLARIZATION IN THE NUCLEON

We make a first crude direct estimate of the net gluon polarization in the proton, ΔG, combining data on the asymmetries in high-pT hadron production from the HERMES, SMC and COMPASS collaborations. Although these data sample a restricted range of x, they provide no hint that ΔG is large. Fixing the normalizations of different theoretical parametrizations using the hadron asymmetry data, we find typical central values of ΔG ~ 0.5, with uncertainties of similar magnitude. Values of ΔG≥2 are disfavored by Δχ2 ~ 9 to 20, depending on the parametrization used.

Extracting the Sivers function from polarized semi-inclusive deep inelastic scattering data and making predictions

The most recent data on the weighted transverse single spin asymmetry ${A}_{UT}^{\mathrm{sin}({\ensuremath{\phi}}_{h}\ensuremath{-}{\ensuremath{\phi}}_{S})}$ from HERMES and COMPASS Collaborations are analyzed within the LO parton model with unintegrated parton distribution and fragmentation functions; all transverse motions are taken into account, with exact kinematics, in the elementary interactions. The overall quality of the data is such that, for the first time, a rather well constrained extraction of the Sivers function for $u$ and $d$ quarks is possible and is performed. Comparisons with models are made. Based on the extracted Sivers functions, predictions for ${A}_{UT}^{\mathrm{sin}({\ensuremath{\phi}}_{h}\ensuremath{-}{\ensuremath{\phi}}_{S})}$ asymmetries at JLab are given; suggestions for further measurements at COMPASS, with a transversely polarized hydrogen target and selecting favorable kinematical ranges, are discussed. Predictions are also presented for single spin asymmetries in Drell-Yan processes at the Relativistic Heavy Ion Collider and the Facility for Antiproton and Ion Research at GSI-Darmstadt.

Hadronization in polarized semi-inclusive DIS: the question of independent fragmentation

The new formalism for description of (un)polarized semi-inclusive DIS at moderate energies is developed. Hadron production is modeled as a product of distribution functions and hadronization functions (HFs) weighted by the hard scattering cross sections, as it is done in the {\tt LEPTO} event generator. The correct treatment of polarization effects shows that description of semi-inclusive DIS within this formalism includes a new nonperturbative input -- {\it polarized} HFs. It is shown that this approach does not correspond to the commonly adopted one with the independent quark fragmentation. The purity method used by the HERMES collaboration mixes up the two approaches and ignores the contributions from polarized HFs. This method cannot be considered a precise tool for the extraction of polarized quark distributions from measured SIDIS asymmetries.

Role of Cahn and Sivers effects in deep inelastic scattering

The role of intrinsic $\bfk_\perp$ in inclusive and semi-inclusive Deep Inelastic Scattering processes ($\ell p \to \ell h X$) is studied with exact kinematics within QCD parton model at leading order; the dependence of the unpolarized cross section on the azimuthal angle between the leptonic and the hadron production planes (Cahn effect) is compared with data and used to estimate the average values of $k_\perp$ both in quark distribution and fragmentation functions. The resulting picture is applied to the description of the weighted single spin asymmetry $A_{UT}^{\sin(\phi_\pi - \phi_S)}$ recently measured by the HERMES collaboration at DESY; this allows to extract some simple models for the quark Sivers functions. These are compared with the Sivers functions which succeed in describing the data on transverse single spin asymmetries in $\pup p \to \pi X$ processes; the two sets of functions are not inconsistent. The extracted Sivers functions give predictions for the COMPASS measurement of $A_{UT}^{\sin(\phi_\pi - \phi_S)}$ in agreement with recent preliminary data, while their contribution to HERMES $A_{UL}^{\sin\phi_\pi}$ is computed and found to be small. Predictions for $A_{UT}^{\sin(\phi_K - \phi_S)}$ for kaon production at HERMES are also given.

THE POLARIZED NUCLEON SEA IN THE MESON CLOUD MODEL

We report a study on the non-pertribative structure of the polarized nucleon sea using the meson cloud model. It is found that the SU (2) flavour symmetry and strange-antistrange symmetry in the polarized nucleon sea are broken. Our calculations are in agreement with the recent results from HERMES Collaboration.

Hadron attenuation in deep inelastic lepton-nucleus scattering

We present a detailed theoretical investigation of hadron attenuation in deep inelastic scattering (DIS) off complex nuclei in the kinematic regime of the HERMES experiment. The analysis is carried out in the framework of a probabilistic coupled-channel transport model based on the Boltzmann-Uehling-Uhlenbeck (BUU) equation, which allows for a treatment of the final-state interactions (FSI) beyond simple absorption mechanisms. Furthermore, our event-by-event simulations account for the kinematic cuts of the experiments as well as the geometrical acceptance of the detectors. We calculate the multiplicity ratios of charged hadrons for various nuclear targets relative to deuterium as a function of the photon energy nu, the hadron energy fraction z_h=E_h/nu and the transverse momentum p_T. We also confront our model results on double-hadron attenuation with recent experimental data. Separately, we compare the attenuation of identified hadrons (pi^\pm, \pi^0, K^\pm, p and pbar) on Ne and Kr targets with the data from the HERMES Collaboration and make predictions for a Xe target. At the end we turn towards hadron attenuation on Cu nuclei at EMC energies. Our studies demonstrate that (pre-)hadronic final-state interactions play a dominant role in the kinematic regime of the HERMES experiment while our present approach overestimates the attenuation at EMC energies.